1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus, and more particularly, to an image fixing apparatus to fix an image onto a printing medium, an image forming apparatus having the same, and an image fixing temperature control method thereof.
2. Description of the Related Art
An image forming apparatus, such as, for example, a multi-function image forming apparatus that combines various functions such as a copier, a printer, or a facsimile, mainly operates to recreate an input image signal onto a printing medium. An image forming apparatus generally includes a paper feeder to feed a printing medium, a developer to develop an electrostatic latent image on a photoconductive medium, a transfer mechanism to transfer the developed image onto the printing medium, and an image fixing apparatus to fix the transferred image in the printing medium. An example of a conventional image fixing apparatus is illustrated in FIG. 1.
Referring to FIG. 1, a conventional image fixing apparatus 1 of an image forming apparatus includes a heating roller 10, which houses a heater 11 therein, and a pressing roller 20, which is pressed towards the heating roller 10. A temperature sensor 30 is installed on the outer circumference of the heating roller 10 to measure the temperature of the heating roller 10.
A thermal sensor (such as a thermistor or thermostat) is generally used as the temperature sensor 30. The temperature sensor 30 detects the surface temperature of the heating roller 10, generally from a spot near the middle part of the heating roller 10. Therefore, the temperature of the heater 11 is controlled according to the result of the sensing by the temperature sensor 30.
However, because the temperature sensor 30 is installed on the outer surface of the heating roller 10 while the heater 11 is inside the heating roller 10, it is difficult to detect accurately the temperature of the heater 11. Accordingly, it is difficult to control the temperature of the heater 11 accurately, and bad image fixation may occur due to overheating or underheating of the heater 11.
FIG. 2 shows a temperature variation along an axis of the roller 10 both at an early stage in an operation of the image fixing apparatus (below T0) and at a later stage of the operation of the image fixing apparatus (above T0). In FIG. 2, S.P. represents a position at or near a midpoint of the axis where the temperature sensor 30 detects the temperature of the heater 11. T1 represents the early stage temperature variation in the absence of control and T2 represents the early stage temperature variation, where the temperature is controlled to bring the temperature at S.P. up to the reference temperature T0. T3 represents the later stage temperature variation in the absence of control and T4 represents the later stage temperature variation, where the temperature is controlled to bring the temperature at S.P. down to the reference temperature T0. Because the temperature sensor 30 detects the temperature at only one spot (S.P) at or near the middle part of the heating roller 10, as illustrated in FIG. 2, and since the middle (S.P.) of the roller 10 tends to heat up more quickly than the ends of the roller, a large temperature gap (ΔT1) is generated at the ends of the roller 10 between the previously fixed reference temperature (T0) and the controlled temperature (T2) at the early stage operation, even when the middle of the roller has reached the reference temperature T0. Similarly, since the middle S.P. of the roller 10, tends to lose heat more quickly than the ends of the roller 10, a large temperature gap (ΔT2) is generated at the ends of the roller 10 between the previously fixed reference temperature (T0) and the controlled temperature (T4) after a long period of operation.
In particular, a large temperature gap (ΔT1) occurs between the lowest value of the controlled early stage temperature of the early stage temperature (T2) and the predetermined image fixing reference temperature (T0). A large temperature gap (ΔT2) also occurs between the highest value of the controlled long time operation temperature (T4) and the predetermined image fixing reference temperature (T0). The temperature gaps (ΔT1, ΔT2) cause problems such as over-fixing, under-fixing, or shortened lifespan.
It has thus been proposed that the first image fixing on the printing medium P by the image fixing apparatus 1 be shortened by reducing the heating volume so that the heater 11 can reach a desired high temperature to heat the heating roller 10 within a short time. In this case, however, if the temperature is sensed inaccurately by the temperature sensor 30, the heater 11 may be damaged.